π-Conjugated Chelating Polymers with a Charged Iridium Complex

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J. Phys. Chem. C 2007, 111, 1166-1175

π-Conjugated Chelating Polymers with a Charged Iridium Complex in the Backbones: Toward Saturated-Red Phosphorescent Polymer Light-Emitting Diodes Shu-Juan Liu,† Qiang Zhao,‡ Yun Deng,‡ Yi-Jie Xia,‡ Jian Lin,‡ Qu-Li Fan,*,† Lian-Hui Wang,‡ and Wei Huang*,†,‡ Institute of AdVanced Materials (IAM), Nanjing UniVersity of Posts and Telecommunications (NUPT), 66 XinMoFan Road, Nanjing 210003, People’s Republic of China, and Institute of AdVanced Materials (IAM), Fudan UniVersity, 220 Handan Road, Shanghai 200433, People’s Republic of China ReceiVed: July 28, 2006; In Final Form: October 31, 2006

A series of π-conjugated chelating polymers with charged iridium (Ir) complex units based on 1,10phenanthroline in the backbones were synthesized by Suzuki polycondensation, and homogeneous polymeric phosphorescent materials were obtained. 3,8-Dibromo-1,10-phenanthroline served as an N∧N ligand to form a charged Ir complex monomer with 1-(9,9-dioctylfluorene-2-yl)isoquinoline (Fiq) as the cyclometalated ligands. Chemical and photophysical characterization confirmed that the Ir complex was incorporated into the backbones as one of the repeat units. Chelating polymers showed efficient energy transfer from the host to the guest in the solid state, and almost complete energy transfer was realized when the feed ratio of the Ir complex monomer was 4 mol %. All chelating polymers displayed good thermal stability, redox reversibility, and film-forming properties. Polymer light-emitting diodes using chelating polymer without carbazole units (phen-PFOIr2) and chelating polymer with carbazole units (phen-PFOCzIr2) as the light-emitting layers were fabricated, and saturated-red electrophosphorescence was achieved.

Introduction Recently, charged iridium (Ir) complexes have attracted increasing attention because they provide emission centers and ionic conductivity in the same species and have some features that might make them one of the best candidates for lighting, display, and light-emitting electrochemical cell (LEC) applications.1 First, the synthetic conditions of charged Ir complexes are much milder than those of neutral ones.2 Next, contrary to conventional organic light-emitting diodes (OLEDs) based on neutral complexes requiring low-work-function cathodes, charged Ir complexes show efficient operation even with air-stable cathodes. Third, further improving the stability of devices can be expected because of their excellent redox stabilities. Moreover, charged Ir complexes offer the property of charge transfer, thus lowering the power consumption of devices. Slinker et al.3 first demonstrated efficient non-doped single-layer devices emitting yellow light with a brightness exceeding 300 cd/m2 and a luminous power efficiency exceeding 10 lm/W at just 3 V using Au for the electrodes. Recently, they also fabricated non-doped single-layer devices emitting green light based on the charged Ir complexes [Ir(F-mppy)2(dtb-bpy)](PF6).4 With a Au top contact, the luminance reached 110 cd/m2, and the quantum efficiency reached 1.1% at -3 V. With a CsF/Al top contact, the luminance and quantum efficiency at +3 V were 170 cd/m2 and 1.8%, respectively. Non-doped single-layer devices emitting blue-green light based on charged Ir complexes have also been reported.5 With a Au top contact, the luminance and quantum efficiency were 33 cd/m2 and 0.75% at -6 V, * Corresponding author. Tel.: +86 25 8349 2333. Fax: +86 25 8349 2333. E-mail: [email protected] (W.H.), [email protected] (Q.L.F.). † Nanjing University of Posts and Telecommunications (NUPT). ‡ Fudan University.

and with a CsF/Al top contact, the luminance and quantum efficiency were 40 cd/m2 and 0.16% at only 3.5 V. It is well-known that non-doped Ir complexes lead to triplettriplet (T-T) annihilation and concentration quenching. Therefore, host materials are necessary. Plummer et al.6 investigated the characteristics of electrophosphorescence devices based on charged Ir complexes doped into poly(N-vinylcarbazole) (PVK) and obtained devices with a high efficiency of up to 23 cd/A. However, a problem with phase separation in the doping systems was encountered. The problem of phase separation is more serious for charged Ir complexes than for systems based on neutral Ir complexes because of the poor compatibility between pure charged complexes and hydrophobic host materials. To solve the problem of phase separation, the incorporation of Ir complexes into the molecular structure of host materials by covalent bond has been investigated and found to be an ideal technique.7-11 Holmes et al.9 first reported fluorene oligmers and polymers containing neutral Ir complexes in the main chains. The Ir units were covalently attached to and in conjugation with fluorene oligmer and polymer. Their investigations showed that the polymer complexes have better triplet energy-level matching, and these covalently bond materials exhibit improvements in efficiency over simple blends, thus providing efficient red phosphorescent polymer light-emitting diodes. In addition, the synthesis and photophysical properties of novel monoterpyridine-poly(ethylene glycol) (PEG)-, poly(ethylene oxide) (PEO)-, and poly(styrene) (PS)-functionalized charged Ir complexes have been reported.12,13 However, no energy or charge transfer occurred in these systems containing charged Ir complexes because the main chains were nonconjugated and the charged Ir complexes were only on the end group. Therefore, it would be interesting to investigate π-conjugated chelating polymers containing a charged Ir complex as a repeat unit incorporated

10.1021/jp064819w CCC: $37.00 © 2007 American Chemical Society Published on Web 12/16/2006

Toward Saturated-Red Phosphorescent PLEDs into the main chain. Two roles are played by the conjugated segment: polymer ligand and host for the energy-transfer system. As one of the three primary colors for full color application, saturated-red emission is indispensable. The performances of red-emitting devices, however, need to be improved, particularly compared to those green-emitting devices in terms of efficiency and color purity. In our previous work,14 π-conjugated chelating polymers based on fluorene with a charged Ir complex covalently attached to backbones were synthesized. In these chelating polymers, 2,2′-bipyridine (bpy) was used as the N∧N ligand of the Ir complex. Efficient energy transfer was realized, and these chelating polymers gave red phosphorescence in the solid state. However, saturated-red phosphorescence was not achieved. It was reported that the bpy in the backbones of the conjugated polymers was not coplanar because of ortho hydrogen interactions,15 reducing the conjugated length of the N∧N ligand (bpy) and consequently influencing the emission wavelength of the Ir complex in our previous work. In this article, we successfully synthesized a new series of π-conjugated polymers with a charged Ir complex in the backbones. In these chelating polymers, 1,10-phenanthroline (phen) was used instead of bpy as the N∧N ligand because the configuration of phen is coplanar and phen has a larger conjugated length than bpy. Hence, longer emission wavelengths of the corresponding Ir complex can be expected. Fluorene segments were selected as the host because of their high fluorescence, charge-transport properties, and good chemical and thermal stability.16 Carbazole (Cz) units were also introduced into the backbones of the polymer. Here, Cz plays multiple roles: as a hole-transport moiety on the polymer/ITO interface, as a charge-trapping site, and as a barrier for energy backtransfer from the triplet state of the Ir complex to that of the host.8,17-19 Photophysical properties, energy transfer from host to guest, and electrochemical properties were investigated. Devices based on chelating polymers without carbazole units (phen-PFOIr2) and chelating polymers with carbazole units (phen-PFOCzIr2) were fabricated, and saturated-red electrophosphorescence was achieved. To the best of our knowledge, this is the first report of polymer light-emitting diodes (PLEDs) with saturated-red emission based on chelating polymers with a charged Ir complex in the backbones. Experimental Section Synthesis. All manipulations involving air-sensitive reagents were performed in an atmosphere of dry N2. The solvents (THF, toluene) were purified by routine procedures and distilled under dry N2 before use. All reagents, unless otherwise specified, were purchased from Aldrich, Acros, and Lancaster and were used as received. 9-Octyl-3,6-dibromocabazol (M4) was synthesized according to the method reported previously.20 2-Bromo-9,9-dioctylfluorene (1). To a mixture of 2-bromofluorene (4.5 g, 20 mmol) and KOH (11.2 g, 200 mmol) in DMSO (10 mL) was added by syringe 1-bromooctane (8.5 g, 44 mmol). The resulting solution was stirred at 60 °C overnight. The resulting mixture was poured into H2O (200 mL) and then was extracted three times with ethyl acetate. The combined organic layers were washed with brine and dried over anhydrous MgSO4. The solvent was removed under reduced pressure. The crude product was purified by column chromatography using hexane as the eluent (Rf ) 0.8) to yield a colorless oil (8.84 g, 94%). 1H NMR (400 MHz, CDCl3): δ (ppm) ) 7.65 (m, 1H), 7.54 (m, 1H), 7.44 (m, 2H), 7.31 (m, 3H), 1.92 (m, 4H), 0.97-

J. Phys. Chem. C, Vol. 111, No. 3, 2007 1167 1.24 (m, 20H), 0.81 (t, J ) 7.2 Hz, 6H), 0.58 (m, 4H). Anal. Calcd (%) for C29H41Br: C 74.18, H 8.80. Found: C 74.48, H 8.82. 9,9-Dioctylfluorene-2-boronic Acid (2). To a solution of 1 (10 g, 21.3 mmol) in THF (80 mL) at -78 °C was added n-butyl lithium in hexane (1.6 M, 14.6 mL, 23.4 mmol) dropwise. After 45 min, trimethyl boronate (5.5 g, 53.3 mmol) was added by syringe. Then, the resulting mixture was allowed to warm slowly to room temperature and was stirred overnight. HCl (2 N, 100 mL) was added to the stirred solution as the solution was maintained at 0 °C for 1 h. The organic layer was separated, and the water layer was extracted with diethyl ether. The combined ether layers were washed twice with brine (100 mL) and then dried over anhydrous MgSO4. The solvent was removed under reduced pressure. The crude product was purified by column chromatography using a mixture of hexane and ethyl acetate (3:1 v/v) as the eluent (Rf ) 0.6) to yield a white solid (6.79 g, 73.4%). 1H NMR (400 MHz, CDCl3): δ (ppm) ) 8.31 (m, 1H), 8.22 (s, 1H), 7.89 (d, J ) 7.2 Hz, 1H), 7.81 (m, 1H), 7.38 (m, 3H), 2.09 (m, 4H), 0.97-1.22 (m, 20H), 0.77 (t, J ) 7.2 Hz, 6H), 0.68 (m, 4H). Anal. Calcd (%) for C29H43O2B: C 80.17, H 9.98. Found: C 80.04, H 9.75. 1-(9,9-Dimethylfluorene-2-yl)isoquinoline (Fiq) (3). Tetrakis(triphenylphosphine) palladium (0.425 g, 0.37 mmol) was added to a mixture of 1-chloroisoquinoline (1.85 g, 11.5 mmol), 2 (5 g, 11.5 mmol), toluene (50 mL), ethanol (25 mL), and 2 M sodium carbonate aqueous solution (25 mL) under vigorous stirring. The mixture was stirred at 70 °C for 24 h under N2 atmosphere. After being cooled to room temperature, the reaction mixture was poured into water and then extracted with ethyl acetate. The organic layer was washed with brine several times. Then, the solvent was evaporated. The product thus obtained was purified by silica gel column chromatography using a mixture of hexane and ethyl acetate (9:1 v/v) as the eluent (Rf ) 0.5) to yield a colorless oil (5.0 g, 83.9%). 1H NMR (400 MHz, CDCl3): δ (ppm) ) 8.65 (d, J ) 5.6 Hz, 1H), 8.10 (d, J ) 8.8 Hz, 1H), 7.85-7.92 (m, 2H), 7.78 (m, 1H), 7.64-7.73 (m, 4H), 7.52 (m, 1H), 7.30-7.40 (m, 3H), 2.01 (m, 4H), 1.02-1.21 (m, 20H), 0.80 (t, 6H), 0.72 (m, 4H). Anal. Calcd (%) for C38H47N: C 88.15, H 9.15, N 2.71. Found: C 88.41, H 9.20, N 2.57. 3,8-Dibromo-1,10-phenanthroline (4).21 A solution of 1,10phenanthroline (5 g, 22 mmol) in nitrobenzene (10 mL) was heated to 130-140 °C in a three-neck flask (50 mL), and bromine (1.7 mL, 32 mmol in 5 mL nitrobenzene) was added dropwise over a period of 30 min. After being stirred for 5 h, the reaction mixture was cooled to room temperature and then treated with concentrated ammonium hydroxide (50 mL), extracted with dichloromethane (3 × 20 mL), and dried over anhydrous MgSO4. The solvent was removed under reduced pressure. The crude product was purified by column chromatography using dichloromethane as the eluent (Rf ) 0.5) to give a white solid (0.5 g, 7%). 1H NMR (400 MHz, CDCl3): δ (ppm) ) 9.21 (d, J ) 2.2 Hz, 2H), 8.44 (d, J ) 2.2 Hz, 2H), 7.78 (s, 2H). Anal. Calcd (%) for C12H6N2Br2: C 42.64, H 1.79, N 8.29. Found: C 42.28, H 1.91, N 8.53. [Ir(Fiq)2(BrphenBr)](PF6) (M3). IrCl3‚3H2O (2.0 g, 5.5 mmol) and 3 (5.7 g, 11.0 mmol) were heated in a mixture of 2-ethoxyethanol and water (40 mL, 3:1 v/v) under N2 atmosphere. This slurry was stirred at 110 °C for 24 h. After being cooled to room temperature, the precipitate was filtered off and washed with water and ethanol to yield red solid 5. CH2Cl2 and methanol (30 mL, 2:1 v/v) were added to a mixture of 5 (0.4 g, 0.2 mmol) and 4 (0.1 g, 0.3 mmol) under N2 atmosphere,

1168 J. Phys. Chem. C, Vol. 111, No. 3, 2007 and the reaction mixture was refluxed for 4 h. After being cooled to room temperature, a 5-fold excess of KPF6 was added, and the mixture was stirred for another 1 h. The solvent was removed, the solid was redissolved in CH2Cl2 (20 mL), the precipitate was filtered off, and methanol was layered on the filtrate. Red crystals of [Ir(Fiq)2(BrbpyBr)](PF6) were recrystallized from the solution (6.5 g, 70%). 1H NMR (400 MHz, DMSO-d6): δ (ppm) ) 9.29 (m, 2H), 8.98 (d, J ) 8.0 Hz, 2H), 8.38 (m, 4H), 8.16 (d, J ) 7.2 Hz, 2H), 7.94 (m, 4H), 7.84 (m, 2H), 7.50-7.61 (m, 4H), 7.41 d, J ) 7.2 Hz, 2H), 7.26 (t, J ) 7.2 Hz, 2H), 7.13 (T, J ) 7.6 Hz, 2H), 6.82 (d, J ) 7.6 Hz, 2H), 6.56 (s, 2H), 1.89-2.21 (m, 8H), 0.89-1.01 (m, 40H), 0.51-0.66 (m, 20H). Anal. Calcd (%) for C88H98N4F6PBr2Ir: C 61.86, H 5.78, N 3.27. Found: C 61.81, H 5.84, N 3.53. [Ir(Fiq)2(phen)](PF6) (6). The synthetic method used is similar to that for M3; 72% yield. 1H NMR (400 MHz, CDCl3): δ (ppm) ) 8.95 (m, 2H), 8.66 (m, 2H), 8.30 (s, 2H), 8.22 (s, 2H), 7.93 (m, 4H), 7.82 (m, 4H), 7.61 (m, 2H), 7.32 (m, 2H), 7.24 (m, 4H), 7.21 (m, 2H), 7.13 (t, J ) 6.0 Hz, 2H), 7.02 (d, J ) 6.0 Hz, 2H), 6.72 (s, 2H), 2.04 (m, 8H), 1.10 (m, 40H), 0.64-0.80 (m, 20H). Anal. Calcd (%) for C88H100N4F6PIr: C 68.15, H 6.50, N 3.61. Found: C 68.11, H 6.51, N 3.73. 9,9-Dioctyfluorene (7). To a mixture of fluorene (8.30 g, 50 mmol) and KOH (28 g, 500 mmol) in THF (120 mL) at room temperature was added by syringe 1-bromooctane (21.25 g, 110 mmol). After workup, the mixture was poured into water and extracted with ethyl acetate. The organic extracts were washed with brine and dried over magnesium sulfate. The solvent was removed under reduced pressure. The crude product was purified by column chromatography using hexane as the eluent (Rf ) 0.8) to give a yellow oil (17.58 g, 85%). 1H NMR (400 MHz, CDCl3): δ (ppm) ) 7.83 (dd, J ) 4.3 Hz, 2H), 7.43 (m, 6H), 2.12 (m, 4H), 1.35-1.24 (m, 20H), 0.96 (t, J ) 7.1 Hz, 6H), 0.79 (m, 4H). 9,9-Dioctyl-2,7-dibromofluorene (M2). To a solution of 9,9dioctylfluorene (11.72 g, 30 mmol) in CHCl3 (50 mL) at 0 °C were added ferric chloride (75 mg, 0.46 mmol) and bromine (3.24 mL, 63 mmol). The reaction was allowed to proceed in the dark. The solution was warmed to room temperature and stirred for 3 h. The resulting slurry was poured into water and washed with sodium thiosulfate until the red color disappeared. The aqueous layer was extracted with ethyl acetate (three times), and the combined organic layers were dried over magnesium sulfate. The product was recrystallized from ethanol (15.46 g, 94%). 1H NMR (400 MHz, CDCl3): δ (ppm) ) 7.51 (d, J ) 8.8 Hz, 2H), 7.46 (d, J ) 1.6 Hz, 2H), 7.44 (s, 2H), 1.91 (m, 4H), 1.99-1.22 (m, 20H), 0.83 (t, J ) 7.2 Hz, 6H), 0.57 (m, 4H). 13C NMR (100 MHz, CDCl3): δ ) 152.31, 139.97, 130.04, 126.07, 121.38, 120.96, 55.56, 40.02, 31.63, 29.78, 29.04, 29.01, 23.51, 22.47, 13.94. Anal. Calcd (%) for C29H40Br2: C 63.51, H 7.35. Found: C 63.64, H, 7.53. 9,9-Dioctylfluorene-2,7-bis(trimethylene boronate) (M1). A solution of 2,7-dibromo-9,9-dioctylfluorene (10.97 g, 20 mmol) in dry THF was added slowly under nitrogen to a stirred mixture of magnesium turnings (1.19 g, 50 mmol) in dry THF containing a catalytic amount of iodine to form the Grignard reagent. The Grignard reagent solution was slowly dropped into a stirred solution of trimethyl borate (11.5 mL, 100 mmol) in dry THF at -78 °C over a period of 1.5 h, and the mixture was then slowly warmed to room temperature. The reaction mixture was stirred (vigorous stirring was required to avoid gel formation) at room temperature for 3 days and then poured into crushed ice containing sulfuric acid (5%) under stirring. The mixture

Liu et al. was extracted with ether, and the combined extracts were evaporated to give 9,9-dioctylfluorene-2,7-diboronic acid. The crude acid was washed with hexane to give a white solid (4.305 g, 45%). The diboronic acid was then refluxed with 1,3propandiol (1.24 g, 20 mmol) in toluene for 10 h. After workup, the crude product was recrystallized from hexane to afford 9,9dioctylfluorene-2,7-bis(trimethylene boronate) as white crystals (3.62 g, 72%). 1H NMR (400 MHz, CDCl3): δ (ppm) ) 7.75 (d, J ) 8.4 Hz, 2H), 7.67-7.72 (m, 4H), 4.21 (t, 8H), 2.10 (m, 4H), 1.98 (m, 4H), 0.92-1.23 (m, 20H), 0.81 (t, J ) 7.2 Hz, 6H), 0.52 (m, 4H). Anal. Calcd (%) for C35H52B2O4: C 75.28, H 9.39. Found: C 75.45, H 9.75. General Procedure of Copolymerization by Suzuki CrossCoupling Method. To a mixture of 9,9-dioctylfluorene-2,7-bis(trimethylene boronate) (M1) (1 equiv), dibromo compound [1 equiv, including Ir complex monomer (M3) and 9,9-dioctyl2,7-dibromofluorene (M2) or 9-dioctyl-3,6-dibromocabazol (M4)], tetrabutylammonium bromide, and tetrakis(triphenylphosphine)palladium [Pd(PPh3)4] (4.0 mol %) was added a degassed mixture of toluene ([monomer] ) 0.25 M) and aqueous 2 M potassium carbonate (3:2 v/v). The mixture was vigorously stirred at 85-90 °C for 72 h, and then bromobenzene was added. After the mixture had cooled to room temperature, it was washed with water. The solution was evaporated, and then it was slowly dropped into 220 mL of a mixture of methanol and deionized water (10:1 v/v). A fibrous solid was obtained by filtration. The solid was dissolved, and then the evaporated solution was dropped slowly into methanol (250 mL). The fibrous solid obtained was filtered and then washed with acetone in a Soxhlet apparatus for 3-5 days to remove oligomers and catalyst residues. The resulting polymers were collected and dried under vacuum. Yields: 55-65%. phen-PFOIr01. 1H NMR (400 MHz, CDCl3): δ (ppm) ) 7.67-7.85 (m, 6H), 2.12 (m, 4H), 1.05-1.28 (m, 20H), 0.630.89 (m, 10H). 13C NMR (100 MHz, CDCl3): δ (ppm) ) 152.05, 140.73, 140.25, 126.38, 121.72, 120.20, 55.57, 40.62, 32.02, 30.27, 29.45, 24.14, 22.83, 14.30. Anal. Calcd (%): C 89.54, H 10.36, N 0.04. Found: C 89.10, H 10.48, N 0.10. phen-PFOIr05. 1H NMR (400 MHz, CDCl3): δ (ppm) ) 7.67-7.85 (m, 6H), 2.12 (m, 4H), 1.05-1.28 (m, 20H), 0.630.89 (m, 10H). 13C NMR (100 MHz, CDCl3): δ (ppm) ) 152.05, 140.74, 140.26, 126.40, 121.72, 120.21, 55.57, 40.63, 32.03, 30.27, 29.46, 24.15, 22.83, 14.30. Anal. Calcd (%): C 89.21, H 10.30, N 0.07. Found: C 89.14, H 10.16, N 0.12. phen-PFOIr2. 1H NMR (400 MHz, CDCl3): δ (ppm) ) 7.67-7.85 (m, 6H), 2.12 (m, 4H), 1.05-1.28 (m, 20H), 0.630.89 (m, 10H). 13C NMR (100 MHz, CDCl3): δ (ppm) ) 152.05, 140.72, 140.26, 126.40, 121.72, 120.21, 55.58, 40.63, 32.04, 30.28, 29.47, 24.15, 22.85, 14.31. Anal. Calcd (%): C 88.02, H 10.07, N 0.27. Found: C 87.60, H 10.14, 0.33. phen-PFOIr4. 1H NMR (400 MHz, CDCl3): δ (ppm) ) 7.67-7.85 (m, 6H), 2.12 (m, 4H), 1.05-1.28 (m, 20H), 0.630.89 (m, 10H), 8.95 (m, 3.5% × 4H, ArH of Ir complex), 8.63 (m, 3.5% × 4H, ArH of Ir complex), 8.28 (m, 3.5% × 2H, ArH of Ir complex). 13C NMR (100 MHz, CDCl3): δ (ppm) ) 152.04, 140.75, 140.25, 126.38, 121.73, 120.16, 55.57, 40.58, 31.99, 30.25, 29.42, 24.14, 22.79, 14.23. Anal. Calcd (%): C 86.58, H 9.81, N 0.52. Found: C 86.11, H 9.95, N 0.61. phen-PFOCzIr2. 1H NMR (400 MHz, CDCl3): δ ) 8.43 (m, 2H), 7.44-7.78 (m, 10H), 4.32 (m, 2H), 2.06 (m, 4H), 1.88 (m, 2H), 1.20-1.50 (m, 6H), 0.95-1.11 (m, 20H), 0.67-0.82 (m, 13H). 13C NMR (100 MHz, CDCl3): δ ) 151.92, 141.07, 140.63, 139.78, 133.45, 126.82, 125.88, 123.87, 121.90, 120.09, 119.68, 109.05, 55.89, 43.89, 40.85, 32.01, 31.85, 30.32, 29.45,

Toward Saturated-Red Phosphorescent PLEDs

J. Phys. Chem. C, Vol. 111, No. 3, 2007 1169

SCHEME 1. Synthetic Routes and Molecular Structures of Monomers and Chelating Polymers

1170 J. Phys. Chem. C, Vol. 111, No. 3, 2007

Liu et al.

Figure 2. Thermal gravimetric analysis (TGA) curves of the chelating polymers under a nitrogen atmosphere obtained at a heating rate of 10 °C/min. Figure 1. 1H NMR spectra of the chelating polymers.

TABLE 1: Molecular Weights, Polydispersity Indexes, and Compositions of the Polymers Ir complex content (mol %) polymer

Mw (×103)

PDI

feed

PFO phen-PFOIr01 phen-PFOIr05 phen-PFOIr2 phen-PFOIr4 phen-PFOCzIr2

12.1 50.5 27.2 32.5 11.3 14.4

1.81 2.47 1.96 1.93 1.48 1.57

0.1 0.5 2 4 2

a

copolymera

3.5

Estimated from 1H NMR and elemental analysis data.8

29.28, 27.24, 24.12, 22.79, 22.76, 14.22. Anal. Calcd (%): C 86.69, H 9.08, N 2.27. Found: C 86.15, H 10.01, N 2.36. Characterization. UV-visible absorption spectra were recorded using a Shimadzu 3000 UV-vis-NIR spectrophotometer. NMR spectra were recorded on a Mercury Plus 400 MHz NMR spectrometer. The element analyses were performed on a Vario EL III O-Element Analyzer system. MALDI-TOF experiments were carried out using a Shimadzu AXIMA-CFR plus matrix-assisted laser desorption/ionization time-of-flight mass spectrometer (Kratos Analytical, Manchester, U.K.). Photoluminescence spectra were obtained on an Edinburgh LFS920 fluorescence spectrophotometer. Emission lifetimes were recorded on a single photon counting spectrometer from Edinburgh Instruments (FLS920) with a hydrogen-filled pulse lamp as the excitation source. The data were analyzed by iterative convolution of the luminescence decay profile with the instrument response function using a software package provided by Edinburgh Instruments. Measurement of the absolute photoluminescence (PL) efficiency was performed on a LabsphereIS-080(8′′) instrument that contained an integrating sphere coated on the inside with a reflecting material of barium sulfate; the diameter of the integrating sphere was 8 in. The wavelength range studied was from 400 to 700 nm. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were performed on Shimadzu DSC-60A and DTG-60A thermal analyzers under nitrogen atmosphere at a heating rate of 10 °C/ min. Cyclic voltammetry (CV) was performed on an Eco chemie Autolab apparatus and was performed with a film coated on the working electrodes in a solution of Bu4NPF6 (0.10 M) in acetonitrile at a scan rate of 100 mV/s. It was conducted at room temperature in a typical three-electrode cell with a working electrode of glassy carbon, a reference electrode of Ag/Ag+

Figure 3. Differential scanning calorimetry (DSC) traces of the chelating polymers measured in a nitrogen atmosphere at a heating rate of 10 °C/min.

[referenced against ferrocene/ferrocenium (FOC)], and a counter electrode of Pt wire under a nitrogen atmosphere. The onset potentials were determined from the intersection of two tangents drawn at the rising current and background current of the cyclic voltammogram. The gel permeation chromatography (GPC) analysis of the polymers was conducted on a Shimadzu 10A instrument with THF as the eluent and poly(styrene) as the standard. The data were analyzed using a software package provided by Shimadzu Instruments. The device configuration was ITO/PEDOT:PSS (5000 rpm, 1 min)/chelating polymer (3000 rpm, 1 min)/BCP (30 nm)/LiF (10 Å)/Al. The devices were fabricated by spinning the emissive layer on the top of the ITO/PEDOT:PSS and subsequently evaporating BCP and the metallic top electrode in a home-built evaporation chamber. The concentration of chelating polymer in toluene was 6 mg/mL. Results and Discussion Synthesis of the Ir Complex and Chelating Polymers. The synthetic routes and chemical structures of the chelating polymers are shown in Scheme 1. The Ir complex was synthesized from 1,10-phenanthroline and the corresponding iridium chloride-bridged dimer. The chelating polymers were prepared by Suzuki polycondensation. The feed ratios of Ir complex in the polycondensation were 0, 0.1, 0.5, 2, and 4 mol %, and the corresponding polymers are denoted as PFO, phenPFOIr01, phen-PFOIr05, phen-PFOIr2, and phen-PFOIr4, re-

Toward Saturated-Red Phosphorescent PLEDs

Figure 4. UV-vis absorption spectra of polymer films.

J. Phys. Chem. C, Vol. 111, No. 3, 2007 1171

Figure 6. PL spectra of chelating polymer films excited at 380 nm.

TABLE 2: Photophysical Properties of the Polymers and [Ir(Fiq)2(phen)](PF6) polymer

λmaxa (nm)

ΦPLb (%)

τ1b,c (ns)

τ2b,d (µs)

PFO phen-PFOIr01 phen-PFOIr05 phen-PFOIr2 phen-PFOIr4 phen-PFOCzIr2 bpy-PFOIr2 [Ir(Fiq)2(phen)](PF6)

445 632 632 631 635 638 625 631

40.1 13.8 5.7 4.7 4.2 7.5

4.26 2.54 2.50 2.27 0.77 2.30

1.43 1.44 1.01 0.99 0.98

a Wavelength of maximum of emission, measured at room temperature for a film spun from CHCl3. b ΦPL measured at room temperature for a film spun from CHCl3. c Monitored at λ ) 445 nm. d Monitored at peak emission.

Figure 5. PL spectra of PFO excited at 380 nm and UV-vis absorption spectra of the Ir complex units in film.

spectively. Chelating polymer with charged Ir complex and fluorene and carbazole units simultaneously in the backbones is denoted as phen-PFOCzIr2. The synthesis of chelating polymer bpy-PFOIr2 was reported in our previous work.14 Chelating polymers were characterized by 1H NMR spectroscopy, 13C NMR spectroscopy, and elemental analysis. Figure 1 shows 1H NMR spectra of the chelating polymers. Some H resonances of the Ir complex in the chelating polymers, such as the peaks at δ ≈ 8.95, 8.63, and 8.28 ppm, can be clearly found in the 1H NMR spectrum of the chelating polymer phenPFOIr4, indicating the successful incorporation of the Ir complex into the polymers. However, these peaks cannot be found in the spectra of the other chelating polymers because the of low content of Ir complex. More importantly, the successful incorporation of the Ir complex into the polymers can be further demonstrated by the photophysical properties of the chelating polymers,22 which show the intrinsic absorption and emission peaks assigned to the Ir complex, the intensities of which increased with increasing Ir complex content in the chelating polymers (discussed below). The Ir complex content in the copolymer phen-PFOIr4 was estimated by 1H NMR and elemental analysis data to be about 3.5 mol % (see Table 1), lower than the value for the feed probably because of the reaction activity and/or steric hindrance of the charged Ir complex. The contents of the Ir complex in the other chelating polymers, phen-PFOIr01, phen-PFOIr05, phen-PFOIr2, and phen-PFOCzIr2, could not be estimated because of the limitations on the accuracy of the elemental analysis and the very

weak particular peaks of the Ir complex in the 1H NMR spectra, which were difficult to integrate.8 The weight-average molecular weights of these chelating polymers [estimated by GPC in THF using a calibration curve of poly(styrene) standards] ranged from 11300 to 50500, and their polydispersity indexes (PDIs) ranged from 1.48 to 2.47, consistent with expectation for a Suzuki polycondensation reaction.22 The thermal stability of the chelating polymers in nitrogen was evaluated by thermogravimetric analysis (TGA) (see Figure 2). The decomposition temperatures (Td), with weight losses of 5%, were found to be 415, 390, 390, 395, and 415 °C for phenPFOIr01, phen-PFOIr05, phen-PFOIr2, phen-PFOIr4, and phenPFOCzIr2, respectively. It was reported that poly(9,9-dialkylfluorene) showed a weight loss at about 390 °C in nitrogen.23 Hence, the incorporation of a charged Ir complex into the backbones does not induce obvious changes in Td for the polymers. All of the chelating polymers started weight loss above 390 °C in nitrogen, indicating good thermal stability. This can, in turn, improve the operating lifetime of a device employing such polymers.24 The thermally induced phase transition behavior of the chelating polymers was also investigated by differential scanning calorimetry (DSC) under a nitrogen atmosphere, as shown in Figure 3. Most of the chelating polymers showed a slightly higher Tg value (110-113 °C) than poly(9,9-dialkylfluorene) (∼103 °C).25 For phen-PFOCzIr2, however, no evident phase transition was observed in the heating scan. The amorphous nature and relatively high glass transition temperatures are important for organic/polymer light-emitting materials because these properties help to improve the performance of light-emitting diodes.26

1172 J. Phys. Chem. C, Vol. 111, No. 3, 2007

Figure 7. Relative energy-transfer efficiency as a function of Ir complex content and distance between the host and the guest.

Figure 8. Cyclic voltammograms of [Ir(Fiq)2(phen)](PF6) and chelating polymer films.

Optical Properties. Figure 4 shows the UV-vis absorption spectra of chelating polymer films. The UV-vis spectra of phenPFOIr01 and phen-PFOIr05 are dominated by a single peak with a maximum absorbance at around 385 nm, which is assigned to the absorption peak of the polymer backbones. For phenPFOIr2, weak absorption peaks appear along with the main absorption peak and extend far into the visible region. From the absorption spectra, one can see that the intensity of these peaks increases with increasing Ir complex content in the chelating polymers. These peaks are attributable to metal-toligand charge-transfer (MLCT) transitions of the Ir complex. The observations further demonstrate that the Ir complex is indeed incorporated into the polymers by chemical bonding. For phen-PFOCzIr2, the main absorption peak assigned to the polymer main chains, which is centered at around 345 nm, shows a hypochromatic shift compared to the other chelating polymers with fluorene units as the polymer main chains. This is due to the interruption of the delocalization of the π-electrons along the polymer backbone by the 3,6-carbazole linkage. Photophysical Properties and Energy Transfer. Figure 5 shows the PL spectrum of PFO excited at 380 nm and the UVvis absorption spectrum of the Ir complex unit [Ir(Fiq)2(phen)](PF6) in a film. The absorption spectrum of [Ir(Fiq)2(phen)](PF6) shows broad bands from 270 to 500 nm, with the most intense bands at λ < 300 nm and moderately intense bands at longer wavelengths that extend far into the visible region. The absorption bands at λ < 400 nm are mainly due to spin-allowed π-π* ligand-centered (LC) transitions. The absorption peaks in the visible region are assigned to MLCT transitions. It can be seen from Figure 5 that there is good spectral overlap between the PL emission spectrum of the host polymer PFO and the absorption spectrum of the guest Ir complex [Ir(Fiq)2(phen)]-

Liu et al. (PF6). According to the Fo¨rster mechanism,27-29 the dipoledipole interaction results in efficient transfer of the singlet excited-state energy from the host to the guest. The efficiency of Fo¨rster energy transfer is dependent on the spectral overlap between the host emission and the guest absorption. Therefore, the good overlap of the host polymer emission and guest absorption in this host-guest system can ensure efficient Fo¨rster energy transfer from the host PFO to the guest [Ir(Fiq)2(phen)](PF6). The PL spectra of the chelating polymer films are shown in Figure 6. The blue emission assigned to the π-π* transitions of the host polymer main chains dominates, and the red emission of the guest Ir complex (at around 631 nm) is very weak for the chelating polymer phen-PFOIr01 because the feed ratio of Ir complex monomer (M3) was low and the energy transfer is inefficient. However, the blue emission peaks become weaker, and the red emission peaks simultaneously intensify with increasing the feed ratio of Ir complex monomer. When the feed ratio of the Ir complex monomer is 2 mol %, the red emission dominates. The blue emission of the host is almost quenched completely for polymer phen-PFOIr4, and only the emission from the guest Ir complex is observed, indicating efficient energy transfer from host to guest. These observations suggest that the energy transfer becomes more efficient with increasing Ir complex content in the chelating polymers because the distance between the host molecules and the guest molecules decreases.29 The emission wavelength of the Ir complex centered at 630-635 nm is slightly greater than that of bpy-PFOIr2, probably because of the increased conjugated length of phen compared to bpy. Table 2 reports the photophysical properties of PFO, the chelating polymers, and [Ir(Fiq)2(phen)](PF6). Transient luminescent decays were monitored at λ ) 445 nm (the peak of the blue emission) for PFO and the chelating polymers to further elucidate the energy migration. For PFO, single-exponential decay with an emission lifetime, τ, of 4.26 ns was observed. However, the fluorescent lifetimes recorded at 445 nm were 2.54, 2.50, 2.27, and 0.77 ns for phen-PFOIr01, phen-PFOIr05, phen-PFOIr2, and phen-PFOIr4, respectively, much shorter than that of PFO. Moreover, the emission lifetimes decreased with increasing Ir complex content. The study of Gong et al.30 showed that the decay time of a PFO film with a higher concentration of Ir complex was much shorter than that of a film with a lower concentration, indicating that the energy transfer in the former was more efficient than that in the latter. Therefore, the observation that the emission lifetimes of the chelating polymers monitored at 445 nm are much shorter than that of PFO and become even shorter as the content of the Ir complex increases verifies that energy migration does occur from the host fluorene segments to the guest charged Ir complex units as mentioned above. A higher content of charged Ir complex in the backbone of the polymer will induce more efficient energy transfer from host to guest. Moreover, for these chelating polymers, the observed emission lifetimes recorded at the peak of red emission were 0.98-1.44 µs in the film, revealing the triplet nature of the long-wavelength emission band. The presence of the heavy metal (Ir) ensures rapid intersystem crossing (ISC) to the triplet state in the Ir complex and subsequent emission from this state.30 The lifetimes probably become shorter with increasing Ir complex content because of the quenching associated with molecular packing and triplet-triplet annililation. According to the Fo¨rster energy-transfer mechanism,27,28 a dipole-dipole interaction results in efficient energy transfer from the host to the guest. The rate of Fo¨rster energy transfer (KFET)

Toward Saturated-Red Phosphorescent PLEDs

J. Phys. Chem. C, Vol. 111, No. 3, 2007 1173

TABLE 3: Electrochemical Properties of the Polymers and [Ir(Fiq)2(phen)](PF6) Ered (V)a

Eox (V)a

energy levels (eV)

polymer

Eonset

Epc

Epa

Eonset

Epa

Epc

HOMO

LUMO

Egb

phen-PFOIr01 phen-PFOIr05 phen-PFOIr2 phen-PFOIr4 phen-PFOCzIr2 [Ir(Fiq)2(phen)](PF6)

-2.60 -2.61 -2.61 -2.60 -2.86 -1.60

-2.70 -2.74 -2.72 -2.68 -2.95 -1.78

-2.59 -2.60 -2.60 -2.61 -2.53 -1.58

0.98 0.97 0.96 0.96 0.75 0.76

1.03 1.03 1.03 1.04 1.06 0.88

0.95 0.94 0.95 0.96 0.73 0.80

-5.36 -5.35 -5.34 -5.34 -5.13 -5.14

-1.78 -1.77 -1.77 -1.78 -1.52 -2.78

3.58 3.58 3.57 3.56 3.61 2.36

a

Epa and Epc represent the anodic peak potential and cathodic peak potential, respectively. b Eg represents the band gap energy.

is given by

KFET ) (R0/R)6τd-1

(1)

where R0 is the characteristic Fo¨rster distance; τd is the lifetime of the host when the guest is absent; and R is the distance between the host and the guest, which is written as

R)

-1/3

(4π3 N )

(2)

G

where NG is the Ir complex density. Therefore, the Fo¨rster energy-transfer efficiency from the host to the guest Ir complex is given by

η ) KFET/(KFET + τd-1) ) 1/[1 + (R0/R)6] ) 4π -1/3 6 η ) 1/ 1 + R0/ NG (3) 3

{ [ (

) ]}

All thin films studied here were spin-coated under the same conditions and had approximately the same thickness. Furthermore, all films were excited under the same conditions and had identical absorption and PL spectra. Thus, R0 can be regarded as a constant for all of the chelating polymer films with different Ir complex contents.29 Although the precise value of R0 has not yet been determined for the host-guest system studied here, a value of R0 ≈ 30 Å was obtained from time-resolved studies of Fo¨rster excitation transfer in conjugated polymer blends.29 As a result, the relative energy-transfer efficiency can be predicted as a function of either the Ir complex content or, equivalently, the distance between the host and the guest, as shown in Figure 7. For chelating polymers phen-PFOIr01 and phen-PFOIr05, the content of Ir complex is low, and accordingly, the average distance from the host to the guest is too large (R > R0), so the energy transfer from host to guest is incomplete.32 However, the average distance from the host to the guest decreases with increasing Ir complex content, and consequently, the energy transfer becomes more efficient. For polymer phen-PFOIr4 (for which the feed ratio of the Ir complex monomer is 4 mol %), almost all of the energy can be transferred from the host to the guest Ir complex. However, the PL efficiency is evidently reduced with increasing Ir complex content as a result of T-T annihilation and/or concentration quenching. Therefore, to obtain efficient phosphorescent polymers, the Ir complex content should be controlled appropriately. Electrochemical Properties. The redox behaviors of the charged Ir complex and chelating polymers were investigated by cyclic voltammetry (CV) (see Figure 8). The electrochemical properties are summarized in Table 3. All of the chelating polymers, except for phen-PFOCzIr2, exhibited good reversibility, with onset potentials between around 0.96 and 0.98 V and between -2.60 and -2.61 V vs Ag/AgNO3 reference electrode for the oxidation and reduction processes, respectively.

The good redox reversibility not only allows us to study the electronic properties of the chelating polymers but also indicates that they might be appropriate electrophosphorescent materials for applications in polymer light-emitting diodes. The HOMO/ LUMO energy levels can be estimated using the equation HOMO/LUMO ) -e(Eonset + 4.38) (eV).10,33 Accordingly, the HOMO levels were estimated at between -5.34 and 5.36 eV, and the LUMO levels were estimated at between -1.77 and 1.78 eV. The oxidation and reduction onset potentials for [Ir(Fiq)2(phen)](PF6) were observed at 0.76 and -1.60 V, respectively, and the HOMO and LUMO levels for it were determined to be -5.14 and -2.78 eV, respectively. If the HOMO and LUMO levels of the charged Ir complex chelated into the main chain of the chelating polymer were to undergo no evident change, they would fall within the band gap of the host. As a result, the charged Ir complex chelated into the backbones of polymer would function as both a hole and an electron trap.34 For the chelating polymer with carbazole units (phen-PFOCzIr2), reversible oxidation and reduction processes occur at around 0.75 and -2.86 V, respectively, and the HOMO and LUMO levels are -5.13 and -1.52 eV, respectively. One can see that the incorporation of carbazole units into the chelating polymers gave rise to a HOMO level that was evidently higher than that of the other chelating polymers without carbazole units. From the onset of the absorption peaks, the optical band gap of the chelating polymers were estimated at 2.97-3.11 eV, lower than the results measured by the electrochemical method, although the trends in the variation of the gap values measured by these two different methods are consistent. Electroluminescence Properties. Chelating polymers phenPFOIr2 and phen-PFOCzIr2 showed efficient energy transfer and moderate PL emission efficiencies, so their electroluminescence (EL) properties were investigated. For comparison, devices using bpy-PFOIr2 as the light-emitting layer were also studied. The structures of these devices were ITO/PEDOT:PSS/ chelating polymer/BCP/LiF/Al. The EL spectra are shown in Figure 9. The PL spectra of these chelating polymers show a weak host blue emission, but the EL spectra show only the emission of the guest Ir complex. The complete quenching of the host EL emission is due to the fact that the dominant emission mechanism in phosphorescent PLEDs receives a significant contribution from charge trapping followed by recombination on Ir complex units. The EL emission spectra of phen-PFOIr2 and phen-PFOCzIr2 are almost same as their PL emission spectra, with the peak emission at about 634 nm and a shoulder peak at around 675 nm. The purity of emission color can be defined according to chromaticity coordinates (x, y) determined by Commission Internationale del’ Eclairage (CIE). When the CIE chromaticity coordinates (x, y) are located in the region of “red” in Figure S1 in the Supporting Information, the corresponding emission color is regarded as “saturatedred emission”. The chromaticity value of bpy-PFOIr2 is (x ) 0.58, y ) 0.35), which is near the saturated-red emission (see

1174 J. Phys. Chem. C, Vol. 111, No. 3, 2007

Liu et al. carbazole units into the chelating polymers does not influence the EL spectra, but evidently increases the luminance and current density. Conclusion

Figure 9. Electroluminescence spectra of the chelating polymers. Device structure: ITO/PEDOT:PSS/chelating polymer/BCP/LiF/Al.

In summary, we successfully designed and synthesized a series of saturated-red emission π-conjugated chelating polymers with a charged Ir complex based on 1,10-phenanthroline in the backbones. The energy-transfer, thermal stability, photophysical, and electrochemical properties were investigated. Almost complete energy transfer from the host to the guest Ir complex was achieved in a solid film when the feed ratio was 4 mol %. PLEDs using chelating polymer without carbazole units (phenPFOIr2) and chelating polymer with carbazole units (phenPFOCzIr2) as the light-emitting layers were fabricated, and saturated-red phosphorescence was achieved. The luminance and efficiency of the devices are not satisfying compared to those of devices based on neutral Ir complexes because the devices fabricated here have not been optimized. Greater effort will be made in optimizing the guest-host system, as well as the fabrication and composition of devices. Appropriate guest-host systems can ensure efficient energy transfer from host to guest, thus yielding high-efficiency phosphorescent polymers. On the other hand, the factors influencing the performance of devices based on charged Ir complexes are complicated.3-6 Therefore, the fabrication and composition of such devices will be optimized so as to achieve efficient light-emitting devices based on charged Ir complexes. In addition, investigations of the stability/lifetime of devices based on the light-emitting polymers studied in our work are also undergoing. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China under Grants 60235412, 90406021, and 50428303, as well as the Shanghai Commission of Science and Technology under Grant 04XD14002, Nanjing University of Posts and Telecommunications under Grant NY206070, and the Shanghai Commission of Education under Grant 2003SG03.

Figure 10. Comparison of J-V and L-V curves of devices made using phen-PFOIr2 and phen-PFOCzIr2. Device structure: ITO/PEDOT:PSS/ chelating polymer/BCP/LiF/Al.

Supporting Information Available: CIE 1931 coordinate plot and definition of CIE chromaticity coordinates (x, y) are available free of charge via the Internet at http://pubs.acs.org.

Figure S1). In contrast, for phen-PFOIr2 and phen-PFOCzIr2, saturated-red emission can be achieved with chromaticity values of (x ) 0.61, y ) 0.30) and (x ) 0.67, y ) 0.32), respectively (see Figure S1), which are almost identical to the standard red values of (0.66, 0.34) demanded by the National Television System Committee (NTSC).35 Both bpy and phen are N∧N ligands of Ir complexes, but they have different conjugated lengths. It is well-known that the conjugated length of the ligand influences the emission properties of Ir complexes. Therefore, the purity of the emission colors of chelating polymers based on bpy and phen are different. This is the first investigations into the saturated-red emission of PLEDs based on charged Ir complexes to date.

References and Notes

Figure 10shows the J-V and L-V curves of devices based on chelating polymers phen-PFOIr2 and phen-PFOCzIr2. The turn-on voltages of the two devices are both about 9 V. The current density and luminance are 153.2 mA/cm2 and 83 cd/m2 at 20 V for the device based on polymer phen-PFOIr2 and 265.8 mA/cm2 and 100 cd/m2 at 21 V for the device based on polymer phen-PFOCzIr2. As discussed above, the incorporation of

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